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Abstract:In this study, short fatigue crack initiation and early growth behaviour under low cycle fatigue conditions was investigated in a shot peened low pressure steam turbine blade material. Four different surface conditions of notched samples have been considered: polished, ground, T0 (industry applied shot peened process) and T1 (a less intense shot peened process). Fatigue crack aspect ratio (a/c) evolution in the early stages of crack growth in polished and shot peened cases was found to be quite different: the former was more microstructure dependent (e.g. stringer initiation) while the crack morphology in the shot peened cases was more related to the shot peening process (i.e. surface roughness, position with respect to the compressive stress and strain hardening profiles). Under similar strain range conditions, the beneficial effect of shot peening (in the T0 condition) was retained even at a high strain level (Δε 11 =0.68%), N f, ground < N f, T1 < N f, polished < N f, T0 . The a/c evolution effects were incorporated in K-evaluations and used in calculating da/dN from surface replica data. Apparent residual stress (based on crack driving force ΔK difference) was applied to describe the benefit of shot peening and was seen to extend significantly below the measured residual stress profile, indicating the importance of the strain hardening layer and stress redistribution effects during crack growth.2

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Ni-W-P coatings with different structures were prepared by electroless deposition. The correlation between the coating structure before and after annealing with hardness and corrosion resistance has been examined by microhardness measurement, potentiodynamic polarisation and corrosion weight loss tests, and by quantitative X-ray diffraction technology and scanning electron microscopy observation of the corrosion morphology. The results indicate that incorporation of tungsten (4-5 wt-%) in Ni-P deposits does not change the amorphous structure of the Ni-11?3 P (wt-%) coating or the mixed structure of the Ni-6?8 P (wt-%) coating, but it changes the Ni-5?1 W-9?3 P (wt-%) coating into a critical amorphous structure. The coating with a critical amorphous structure develops a higher degree of crystallisation, larger crystal size and smaller number of Ni 3 P crystallites after annealing at .300uC. It also exhibits the highest peak hardness and the poorest corrosion resistance after annealing at 400uC compared with the other two structural coatings (e.g. amorphous and mixed structures).

A B S T R A C T A numerical prediction of the life of a gas turbine model disc by means of the finite-elementtechnique is presented and the solution is compared with an experimental rim-spinning test. The finite-element method was used to obtain the K solution for a disc with two types of cracks, both at the notch root of the blade insert and located in the corner and in the centre. A crack aspect ratio of (a/c) = 1 was assumed. The fracture mechanics parameters J-integral and K were used in the assessment, which were computed with linear elastic and elastic-plastic material behaviour. Using a crack propagation program with appropriate fatigue-creep crack growth-rate data, previously obtained in specimens for the nickel-based superalloy IN718 at 600 • C, fatigue life predictions were made. The predicted life results were checked against experimental data obtained in real model discs.The numerical method, based on experimental fatigue data obtained in small laboratory specimens, shows great potential for development, and may be able to reduce the enormous costs involved in the testing of model and full-size components. a = crack depth c = half crack length at surface C, m = Paris law parameters E = Young's modulus f = frequency J = J-integral K = stress intensity factor K C = critical stress intensity factor N = number of cycles N t = total number of cycles N p = number of cycles in propagation N i = number of cycles in initiation R = radius r 2 = regression correlation coefficient S UTS = True ultimate tensile stress α = Thermal expansion coefficient K th = Threshold stress intensity factor Correspondence: R.A. Cláudio.